This invention relates to an Li-based hydrogen storage composition and a method of providing a source of hydrogen.
Despite the great interest in metal hydrides for hydrogen storage, Li-based hydrides have so far attracted only limited attention because of their unfavourable hydrogenation thermodynamics Theoretically, lithium is one of the most attractive elements for hydrogen storage. Lithium is the lightest metal, with atomic mass of 6.941, which is almost two times lighter than carbon, and it forms a simple hydride, LiH. Although lithium hydride has an outstanding hydrogen capacity of 12.5 wt. %, until now it has offered no prospects for reversible hydrogen storage. The hydrogen bond in LiH is very strong, xcex94H=xe2x88x9290.5 kJ/mole, and lithium hydride decomposes at temperatures higher than its melting point. The melting temperature of LiH is 680xc2x0 C., and it decomposes only at temperatures above 720xc2x0 C. This feature alone excludes LiH from any practical applications because not only does the hydride melt before decomposition, but also the decomposition temperature is far too high for any reversible hydrogen storage application.
There are only a few known Li-based hydrides, of which LiAlH4 is the most significant. This complex hydride is a common reduction agent and is used in many reactions, for example in the production of borates or silanes, according to the following reactions:
xe2x80x834BCl3+3LiAlH4xe2x86x922B2H6+3LiAlCl4
SiCl4+LiAlH4xe2x86x92SiH4+LiAlCl4
LiAlH4 is not usable for reversible hydrogen storage because it cannot be re-hydrogenated under gaseous hydrogen. After decomposition, LiAlH4 can be re-synthesized only in the course of a chemical reaction, for example:
4LiH+AlCl3xe2x86x92LiAlH4+3LiCl
Therefore, LiAlH4 is not practical for reversible hydrogen storage. The same applies to other known Li-based hydrides, which also are not usable for reversible, gaseous hydrogenation and to date, there are no reports of any Li-based metal hydrides capable of hydrogenation/dehydrogenation cycling under practical hydrogen pressures.
This invention deals with a new generation of Li-based metal hydrides with totally new hydrogenation properties. These hydrides undergo hydrogenation/dehydrogenation cycling under practical conditions of hydrogen pressure and temperature and are thus suitable materials for hydrogen storage.
According to the invention, Li is the basic metallic element which forms complex hydride compositions with other elements. The presence of these other elements causes changes in hydrogen sorption properties of the Li-based hydride, so that the hydride becomes usable for reversible hydrogen storage.
In accordance with one aspect of the invention, there is provided a hydrogen storage composition having a hydrogenated state from which hydrogen is liberated and a dehydrogenated state which absorbs gaseous hydrogen to produce said hydrogenated state, wherein said hydrogenated state comprises:
a hydrided composition of lithium and an element selected from the group consisting of:
a) at least one metal M which forms a hydride, said hydrided composition liberating hydrogen to form a dehydrogenated state comprising LiH and said at least one metal M;
b) at least one element E, said hydrided composition liberating hydrogen to form a dehydrogenated state comprising a compound of lithium and said at least one element E, or a solid solution of lithium and said at least one element E; and
c) at least one metal M and at least one element E, as defined above.
In another aspect of the invention, there is provided a composition as defined hereinbefore, in the hydrogenated state.
In yet another aspect of the invention there is provided a composition as defined hereinbefore, in the dehydrogenated state.
In still another aspect of the invention there is provided a method of producing a source of hydrogen gas comprising liberating hydrogen from a composition of the invention, at an elevated temperature, with formation of the dehydrogenated state of said composition, removing the liberated hydrogen, and regenerating said hydrogenated state by exposing said dehydrogenated state to hydrogen gas.
In another aspect of the invention there is provided a method of producing a hydrogen storage composition of the invention comprising ball milling at least one lithium component selected from elemental lithium and lithium hydride with a component selected from i) at least one metal M, as defined hereinbefore, a hydride thereof, or a mixture thereof; ii) at least one element E, as defined hereinbefore; and iii) at least one of a metal M, as defined hereinbefore, and a hydride thereof; and at least one element E, as defined hereinbefore, to form a lithium-based composition, and, when necessary or desired, hydrogenating the lithium-based composition.